Zinc oxide maximum efficiency length extensional crystals and devices



Oct. 7, 1969 A. R. MOORE 3,471,721

ZINC OXIDE MAXIMUM EFFICIENCY LENGTH EXTENSIONAL CRYSTALS AND DEVICES Filed Oct. 25, 1966 22-- Z/ f g L-22 641/5 6- 6 He. 6 65 azjg j I \\2\ 90 64 63 a P1 A ME I N VE N TOR. flRT/IUR/Q/I/OORE United States Patent 3,471,721 ZINC OXIDE MAXIMUM EFFICIENCY LENGTH EXTENSIONAL CRYSTALS AND DEVICES Arthur R. Moore, Somerset Township, St. Croix County,

Wis., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed Oct. 25, 1966, Ser. No. 589,302 Int. Cl. H01v 7/00 US. Cl. 3109.5 7 Claims ABSTRACT OF THE DISCLOSURE A piezoelectric device is shown comprising at least one element which comprises of a piezoelectric zinc oxide crystal having an axis passing through the crystallographic c-axis and inclined at an angle theta relative to such crystallographic c-axis wherein theta ranges from about 42 to 78 and wherein the crystal has a resistivity between about 0.1 and 10 ohm-cm, and means for making electrode contact with said element on at least one face.

This invention relates to zinc oxide crystals which are capable of propagating length extensional mode with an associated unexpectedly great and heretofore unrealizable electromechanical conversion efliciency, and to the improved devices made therefrom.

Zinc oxide, like quartz, is known to exhibit piezoelectric properties and to have excellent physical and chemical stability. Zinc oxide crystals exhibit the hexagonal Wurtzite lattice structure with the oxygen ions arranged in closest hexagonal packing and the zinc ions occupying half of the tetragonal interstitial positions. Characteristically, the Wurtzite atomic lattice structure has a crystallographic a-plane and a crystallographic c-axis. W. G. Cady in his book Piezoelectricity (published by McGraw-Hill Book, Inc., revised edition, 1964), gives the elastic and piezoelectric constants for this crystal class and describes the piezoelectric modes present for the crystallographic axes.

Heretofore, it was known that zinc oxide single crystals were capable of propagating length extensional modes if such crystals were electroded on faces perpendicular to the c-axis and parallel to the a-plane. However, the electromechanical conversion efficiency of acoustic length extensional modes so generated is less than about I have now discovered, however, that zinc oxide single crystals formed with certain new crystal orientations have coupling coefiicients of acoustic length extensional mode that greatly exceed the characteristic values obtained for those generated along the crystallographic aplane. The fact that these new orientations can be used for generating such efiicient modes is unexpected and surprisingly advantageous since most electromechanical device properties are considered by those skilled in the art to depend upon the square of such coupling coefficients.

The present invention is better understood by reference to the following attached drawings taken together with the associated specification:

FIGURE 1 is an isometric drawing illustrating a section of a crystal of the present invention;

FIGURE 2 is an isometric view of the crystal shown in FIGURE 1 and illustrating the angle 0;

FIGURE 3 is a two-dimensional plot in polar coordinants showing the relationship between 0 and the percent efficiency (coupling coefficient) for a crystal of the invention;

3,471,721 Patented Oct. 7, 1969 FIGURES 4A and 4B illustrates one embodiment of a device of the invention formed from a crystal of FIG- URE 2;

FIGURE 5 illustrates schematically one embodiment of a resonate cavity constructed using a device of FIG- URE 4; and

FIGURE 6 illustrates schematically one embodiment of a quarter wave length transducer constructed using a device of FIGURE 4.

Referring to FIGURE 1 there is seen a section of a large single crystal of zinc oxide which is designated in its entirety by the numeral 100. Such a crystal can be grown by any one of the known processes for the production of single crystal zinc oxide. This crystal is seen to have a crystallographic c-axis and to have three crystallographic c-axes which together define a crystallographic a-plane.

A crystal wafer 10 of the invention can be made by any convenient procedure. For example, such a wafer 10 can be prepared from a crystal 100 by making two spaced parallel cuts through the crystal 100, as follows: The crystal 100 is mounted conveniently on a conventional orientation jig (not shown) using wax or the like. An initial cut (not shown) is made approximately along a crystallographic a-plane near one end or tip of the crystal using a precision wafering machine. This cut tip serves as an orientation guide by subjecting such tip to an X- ray crystal orientation procedure to determine the true direction of the c-axis.

Next, the position of the orientation jig with the crystal mounted therein is carefully adjusted relative to the direction of cut by the blade in the precision wafering machine. One angle selected in this orientation process is the complementary angle to an angle theta (0) (see FIGURE 2). Theta is the angle formed between the crystallographic c-axis and the axis 12 of water 10. The cut crystal axis 12 passes through the crystallographic c-axis and is normal (or orthogonal) to the wafer 10 surface. Theta is measured relative to the crystallographic c-axis. In accordance with this invention, values for the angle theta range from about 42 to 78 and, thus, the complementary angle chosen ranges from about 12 to 48. This relationship between the crystal axis 12 and the crystallographic c-axis conveniently describes what is herein termed the AM series of crystal shapes (see FIGURE 3).

A first preferred range of angles for theta in the AM series is from about 55 to 67 crystals because with these angles, crystals of the invention display maximum acoustic length extensional mode electromechanical coupling efiiciency. A most preferred angle for theta, but one within the AM series, is about 60 because with this angle, a crystal displays greatest acoustic length extensional mode coupling efiiciency.

In such AM series, the electric field propagation direction is required to be at an angle theta to the crystallographic C-axis and to be parallel to such crystal axis 12 in order to obtain the indicated unexpected acoustic wave generation in accordance with the teachings of this invention. Reference to FIGURE 3 is helpful in determining the optimum relationship between the coupling efiiciency and the angle theta in a given crystal. FIGURE 3 indicates the magnitude of the improved coupling achieved in a crystal constructed using the AM series in accordance with the teachings of this invention.

Continuing on with this example of preparing a wafer 10 from crystal 100, if the initial cut through crystal 100 happened to have been made, for example, at an angle 0 which is outside the X-ray machine range, it is sometimes desirable to make a second cut through crystal 100 after the jig has been duly positioned as described. However, after a satisfactory initial out has been made, and

after the crystal 100 in this jig has been thus fixed relative to the cutting direction of the blade in the wafering machine, the orientation jig is advanced a convenient distance, typically ranging from about one-half to four millimeters. A second cut through the crystal 100 is then made so as to separate and define the desired crystal wafer 10. The actual thickness of a crystal wafer is correlated to the end use to which the wafer is to be placed, in accordance with the teachings of this invention, and as hereafter more fully described.

The resulting crystal wafer 10 is preferably subjected to a conventional doping operation (see, for example, the doping procedures described by G. Heiland, E. Mollwo and F. Stockmann in Solid State Physics, vol. 18, pp. 191-319, 1959). If the crystal wafer 10 does not already have a resistivity value between about 0.1 and 10 ohmcm., the crystal resistivity may be adjusted to a value within this range during the doping operation, the exact resistivity value for any given crystal wafer 10 being dependent upon the particular properties desired for the end use intended.

- The resulting doped crystal wafer 10 is now conveniently subjected to a mechanical or chemical polishing operation whereby its opposed faces are polished to a predetermined desired extent, care being exerted to maintain the crystal orientation. Those skilled in the art will appreciate that smooth facial surfaces are desirable in electroding and in subsequent device performance.

Thereafter, the crystal wafer 10 is available for electroding or for mounting in a device. For example, referring to FIGURE 4, there is seen a crystal wafer 18 which has been conveniently formed from a crystal wafer 10, by means of an ultrasonic cutter, grinder, chemical etch bath, or the like. Part A of FIGURE 4 represents a top plan view of wafer 18, while part B thereof represents a vertical sectional view thru wafer 18 taken along the line b-b of FIGURE 4A. The two opposed parallel faces 19 and 20 of the crystal wafer 18 have been appropriately masked and subjected to a vacuum vapor deposition operation so as to form thereon metal electrodes 21 and 22. The use of vapor deposited electrodes is illustrative only, any any suitable method of forming electrodes on a crystal wafer 18 (such as electrolytic or electroless plating, sputtering, conductive pastes, and the like) can be used.

Parallel faces are used on crystal 10 for illustrative purposes only. It is sometimes advantageous to use crystals of this invention having nonparallel faces, or to use crystals of this invention with nonaligned or nonparallel electrodes, or to use such crystals with a plurality of such faces and such electrodes to control production of particular acoustic waves or to control desired electric field propagation. Such constructions and other equivalent ones are within the teachings of the present invention. It will be appreciated that a crystal of the invention can be formed with any desired geometry so long as the angle theta is described.

Observe that, in general, electrodes such as 21 and 22 are each respectively so functionally associated with a crystal Wafer 10 that the direction of electric field propagation is along the axis 12. The axis 12 is related to the crystallographic C-axis thru the angle theta. It is preferred to separate electrodes by a distance measured through the crystal by a distance greater than about 1 millimicron.

Referring once again to FIGURE 3, the plot there shown represents the generalized relationship between percent efliciency and theta in crystals of the invention. In a given crystal wafer an exact statement of the relationship between theta and percent efliciency is not practical because in a given crystal wafer, such as crystal wafer 10 in this description, normal manufacturing variables inherently operate to induce variances in the product crystal. These manufacturing variances typically cause the resulting properties of a length extensional mode generated in a given crystal of this invention to vary slightly from a desired or even optimum condition.

It will be appreciated that the plot shown in FIGURE 3 is based upon the use of an optimum or preferred shape for a crystal of the invention, as described above and as illustrated in FIGURE 2. The plot of FIGURE 3 shows the entire AM series. For reasons similar to those just mentioned in reference to FIGURE 3, minor variations in any actual efficiency curve for a particular crystal of the invention tend to occur.

Surprisingly and unexpectedly, a crystal of this invention of the AM series displays greater electromechanical coupling efficiencies than is obtained by generating an acoustic length extensional mode along the crystallographic C-axis.

FIGURE 5 is a schematic view of a mechanical resonator constructed according to the teachings of W. P. Mason in his book Electro-Mechanical Transducer and Wave Filters (published by D. Van Nostrand Company, Inc., second edition). Metallic electrodes 31 and 32 are adhered to the resonator body 30 which comprises an AM series zinc oxide single crystal prepared according to the teachings of this invention. Electrode 31 is connected to an appropriate signal source 34 and electrode 35 is connected to an appropriate receiver 35. The resulting mechanical resonator has unexpectedly superior bandwidth of transmission when compared to a similar resonator constructed so as to utilize a conventional zinc oxide single crystal adapted to transduce acoustic length extensional waves on the crystallographic c-axis thereof.

FIGURE 6 is a schematic view of a quarter wave transducer constructed according to the teachings of W. P. Mason in his book Electro-Mechanical Transducers and Wave Filters (published by D. Van Nostrand Company, Inc., second edition). Metallic electrodes 61 and 62 are adhered to the resonator body 60 of zinc oxide AM series. The electrodes are connected to an appropriate signal source/receiver 64. The resonator body 60 is adhered to the rigid support 63. The resonator body is coupled to the piston-like motion 65 of the crystal by suitable means, such as oil, elastomer, or the like. Such trans ducers are used for ultrasonic underwater sound echo ranging and communications. The resulting mechanical resonator has unexpectedly superior bandwidth of transmission when compared to a similar device made using a single crystal zinc oxide with orientation on the crystallographic c-axis.

The following examples of particular embodiments are provided to further describe the invention. Each example employs at least one AM series zinc oxide crystal prepared as described above in reference to FIGURE 2. Using these examples devices may be constructed so as to operate on any part of the specified curves in FIG- URE 3.

EXAMPLE I The construction here is identical to that shown in FIGURE 5 and above described.

An AM single crystal zinc oxide wafer, having a resistivity of approximately 10 ohm-cm., is cut with a 1 mm. width, 5 mm. length, and a 0.05 mm. thickness along the AM series with theta equal to 61. The Wafer is electroded with sputtered indium overcoated with vapor deposited gold and has its opposed ends polished flat and perpendicular to the center line thereof. This wafer is mounted for resonance, antiresonance bandwidth measurement on a Q-meter. The bandwidth between the resonant frequency and anti-resonant frequency is one measure of the maximum bandwidth in a crystal filter. Similar measurements are made using a single crystal zinc oxide cut along the crystallographic a-axis. The bandwidth realized for the AM series crystal is approximately 2.6 times that obtained for the crystallographic c-axis crystal.

EXAMPLE II The construction here is identical to that shown schematically in FIGURE 6 and above described.

An AM single crystal zinc oxide wafer, having a resistivity of approximately ohm-cm, is cut with a width of 2 mm., length of 10 mm. and a thickness of 1.0 mm. in the AM series; theta equals 59. The Wafer is polished and electroded as described in Example I. The wafer is adhered to the mounting or supporting medium with a high strength epoxy. Near the resonant frequency (about 470 kilohertz) there is produced a piston-like toand-fro motion at the nonmounted (free) end of the bar. The bandwidth of this quarter wavelength transduced is 26% which is nearly 1.6 times the value obtained for a similar construction using a conventional zinc oxide crystal cut to transducer along the crystallographic caxis.

It will be obvious to those skilled in the art that the high length extensional electromechanical coupling coeflicient of the AM series crystals has many device applications. The devices illustrated in the figures are to be considered illustrative and to be uunderstood as indicating the necessary transmission structures as are well known in the art for the transmission of electromagnetic signals. The foregoing examples are offered as exemplary of the magnitude of possible device designs which depend upon the basic teaching of this invention and are not to be construed as limiting the invention. Various other modifications and embodiments will become apparent to those skilled in the art. However, all such devices, which are characterized in whole or part by the basic phenomenon of the AM series through which this invention had advanced the art are properly considered within the spirit and scope of this invention.

I claim:

1. A piezoelectric device comprising at least one element consisting essentially of a piezoelectric zinc oxide crystal having Wurtzite crystal structure and characterized by being capable of propagating an acoustic length extensional mode with an efficiency greater than that obtained by generating such wave along the crystallographic c-axis, in response to a dynamic electric field applied across said crystal in the direction of acoustic length extensional mode generation, said crystal having both (a) a resistivity between about 0.1 and 10 ohm-cm,

and

(b) an axis passing through the crystallographic c-axis and inclined at an 'angle theta relative to such crystallographic c-axis, theta ranging from about 42 to 78 and means for making electrode contact with said element on at least one face.

2. The piezoelectric device of claim 1 wherein said crystal is further characterized by having maximum length extensional mode coupling efficiency, said crystal further having a value of theta ranging from about to 67.

3. The piezoelectric device of claim 2 wherein theta is about 4. The piezoelectric device of claim 2 in which the smallest dimension of said element corresponds with the inclined axis at an angle theta relative to said crystallographic c-axis and in which electrode contact is made on two face perpendicular to said inclined axis.

5. An electromechanical resonator device comprising (a) a piezoelectric device of claim 4, and

(b) means for sending or receiving the resulting electrical signal generated near the mechanical resonant frequency.

6. An electromechanical transducer device comprising (a) a piezoelectric device of claim 4,

(b) means for sending the electrical signal, and

(c) means for receiving the mechanical signal.

7. An electromechanical transducer device comprising (a) a piezoelectric device of claim 4,

(b) means for sending the mechanical signal, and

(c) means for receiving the electrical signal.

References Cited UNITED STATES PATENTS 2,204,762 6/ 1940 Mason 310- 2,472,691 6/ 1949 Bond 310-95 2,975,354 3/1961 Rosen 3108.1 3,091,707 5/1963 Hutson 3108.1 3,122,662 2/ 1964 Mason 310-95 3,219,850 11/1965 Langevin 310-8.6 3,414,779 12/1968 Bohm 310-8.1 3,388,002 6/1968 Foster 117217 3,397,328 8/ 1968 Schafft 3 l08.1

FOREIGN PATENTS 104,047 6/ 1936 Australia.

366,642 1939 Italy.

355,429 9/ 193-6 Italy.

I. D. MILLER, Primary Examiner U.S. Cl. X.R. 310805, 807 

